Learn Romanian: Când o să devii matur și responsabil? - When are you going to become mature and responsible?

The verb a deveni (to become) is a copulative verb, meaning it directly links the subject to a state or identity. Just like a fi (to be), it connects to a bare noun or adjective in the nominative case, so you don't need any articles.

Why is devii spelled with two is at the end?

The verb a deveni belongs to the same group as its root verb a veni (to come). In the second-person singular (the "you" form), it takes a double -ii ending. The first i is part of the verb's stem, and the second i is the grammatical ending for "you".

What future tense structure is being used with o să devii?

This is the everyday spoken future tense formed by the invariable word o + să + the present subjunctive form of the verb. For the "you" form, the subjunctive is să devii, so the future becomes o să devii (you will become / you are going to become).

How would this sentence change if I were asking a female friend?

Because matur and responsabil are adjectives linked directly to the subject by a deveni, they must agree with the subject in gender and number. For a female subject, you would use the feminine singular forms: Când o să devii matură și responsabilă?